IMPACT OF WATER TREATMENT PLANT RESIDUALS ON FIXED FILM WASTEWATER TREATMENT

In February 2001, the 75th Street Wastewater Treatment Plant (WWTP) in Boulder, Colorado received a large quantity of concentrated ferric chloride residuals from the Boulder Reservoir Water Treatment Plant (BRWTP). The residuals disrupted the biological treatment process, particularly the rock trickling filters (TFs), resulting in an extended period of discharge of poor quality effluent. To protect the biological treatment process at the WWTP, a discharge permit limiting the discharge of total iron to 400 ppd was issued to the water treatment plant.Experiments were conducted to determine the point where WTP residuals would negatively impact WWTP performance and to evaluate the adequacy of the total iron limit. Bench-scale testing was performed on biofilm samples collected from the top of the rock TF. Biofilms were transferred to batch reactors containing primary clarifier effluent that were spiked with a known quantity of BRWTP residuals. The total iron concentrations evaluated ranged from 1.20 to 14.2 mg/L. COD and soluble COD were measured at reactor start-up and then once every thirty minutes.For the BRWTP residual concentrations evaluated, there was no significant impact on soluble COD removal in the batch reactors. Acute iron toxicity was not observed for iron concentrations up to 14.2 mg/L. The 14-day old biofilm samples were more resistant to the effects of iron toxicity than activated sludge samples where performance has been reported to be impacted at iron concentrations as low as 2.0 mg/L. The 75th Street WWTP should be able to routinely receive influent iron concentrations up to 14.2 mg/L without impacting the rock TF performance.